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Motor run caps are designed with only three requirements in mind. First they must impart the appropriate phase shift to auxiliary motor windings dependent on the motor speed requirements. Second they must support high current draw without damage. And third, they must be robust enough to stand large voltage and current spikes (as can occur with step torque loads on the motor) without failure. There are no formal requirements for leakage, capacitance drift, or uniformity throughout the capacitor. The net effect is that many of these caps, while meeting the requirements for their intended use, fail in regard to what we would consider audio performance. This may be due to non-uniform plate material and dielectric properties, inexpensive bonding techniques within the caps, or even non-uniformity in physical construction.

I am not saying that some motor run capacitors can't be used in an audio amp with acceptable results. All that I'm saying is that these caps are not designed for this type of use and you have no way of knowing what tradeoffs the manufacturer used to meet his cost and yield targets. I had a bad experience and after a discussion with the manufacturer decided that it is simply not worth the risk to save a few dollars on an amp. I've used JJ, F&T, Jensen, Sprague/Vishay, and IC all with excellent results at a reasonable cost. So as a matter of personal preference I simply stay away from motor capacitors in anything other then a motor application.

I fully agree, I think attempting to save a few bucks by using motor run caps is a mistake for all but an amp that's thrown together with parts on hand. Spend the extra $$$ and get something designed for the right purpose IMHO. My experience with motor run and start caps is that they're junk. They are designed for one very crude but specific function, and it's NOT filtering - it's power factor correction and the ability to withstand an abusive environment.

Not to interrupt your design discussion but hopefully add to it, one general comment and question about the 300B:

After the cathodes have warmed up and the B+ is applied you can hear the tubes making noise as they thermally expand, and this noise also comes through the speakers. This happens with my Electro-Harmonix 300B Gold tubes at least. My KT88 indirectly heated tubes don't exhibit this behavior.

I'm just wondering if anyone else has the same experience with directly heated power triodes. I'm also trying to figure out what exactly inside the tube it is that's making the noise. Thermal expansion of the plate, of the cathode filament wires rubbing against the supports, etc?

Perhaps better quality tubes don't do this. It stops after 30 seconds or so and is dead silent after that.

Don't worry about the extra filter cap or two, I ordered a couple extra which all arrived yesterday. The only one I don't have is the 10uf. But I'll find a work around or trouble a few tube friends for one.

Don't worry about the exact value. It was just that I needed at least 5.3µf to meet the ripple target and 10µf was the next largest Mundorf cap that was listed on the Parts Connection web page. In reality the 20µf, 30µf, or even the 47µf would all work as well and return even lower ripple numbers. I think that with the 213Ω in front of this capacitor any of these would work without causing an in-rush current problem.

In fact, this will slow the onset of the B+ supply voltage; and since the 300Bs will heat up much faster then the 5U4, we should be safe putting everything on one power switch. Then, by the time the B+ really starts to come up, the 300B filaments will already be hot enough to prevent cathode stripping when the plate-cathode potential field begins to really get strong. Call it a "feature".

Not to interrupt your design discussion but hopefully add to it, one general comment and question about the 300B:

After the cathodes have warmed up and the B+ is applied you can hear the tubes making noise as they thermally expand, and this noise also comes through the speakers. This happens with my Electro-Harmonix 300B Gold tubes at least. My KT88 indirectly heated tubes don't exhibit this behavior.

I'm just wondering if anyone else has the same experience with directly heated power triodes. I'm also trying to figure out what exactly inside the tube it is that's making the noise. Thermal expansion of the plate, of the cathode filament wires rubbing against the supports, etc?

Perhaps better quality tubes don't do this. It stops after 30 seconds or so and is dead silent after that.

sampleaccurate, I would say your assumtion is correct in that you are hearing the expansion of the elements. More specifically, I would suspect the plate. A suggestion I have would be to add a thermal delay-relay to your HV (ie. a 6NO45T - that is the letter O and not a zero). This is a tube that contains a normaly open set of contacts mounted on a bi-metal element and being surrounded by a heating element. Supplied by a 6.3v heater circuit, this tube will have a 45 second delay on closing the contact. It draws a mere 300ma (0.3A) on the heater. By integrating this in to your HV ps section, you can hold off that power for 45 seconds. This should be more than ample to avoid the expansion tings. I would hope that for the lavish cost of the Chinese 300Bs being used in this thread that the manufacturer has taken that consideration. I would hope these tubes would output gold plated sonics at that price.

PS. The delay tube comes in several times and heater voltages. Just like most other VTs, the first number is the heater voltage.The letters are either NO (Normally Open) or NC (Normally Closed) and refer to contact set-up, then the last set of numbers is the amount of delay in seconds. The "T" indicates this is for a 9pin noval socket. No letter means octal pinning. So, the 6NO45T is a 9pin noval tube for 6.3v heaters, Normally Open contacting with 45second delay. Do a Google search and you should be able to find some sources. http://WWW.VACUUMTUBESINC.COM sells these for about $25.

_________________The key to a successful build is to keep the smoke IN the circuit. -Les

This is a little fly in the ointment. I was just getting ready to post the final PS design. It is based on an Edcor XPWR077. But that's not what the customer wants.

So... For the output transformers, thats easy. Go with the Hammond 1630SEA. It meets all requirements and we already considered it in our design. For the primary PS choke, I've already settled on the Hammond 193J 10H choke (or the 193M or 193Q if you can't get the 'J').

The power supply transformer is another matter. Hammond only makes two transformers that meet the PS requirements for this amp. These are the 282X or 382X ( 1000v(500-0-500)@200mA, 5v@3A C.T., 6.3v@6A C.T.). And I couldn't find either of these on the web page you linked. If I need to go to the lower voltage (400-0-400) transformer it will mean an entirely new design from the ground up.

Want to change the PS schematic and re-send. Just so we are all on the same page if we have to fault find. NEVER!!

Well, here is the modified PS schematic as promised. I recalculated the ripple numbers using the 47µf capacitor instead of the 10 µf. I also slightly redrew the schematic because I don't know if the PTFWF460 5V secondary is center tapped or not.

Attachment:

PS Schematics Rev A.png

I'm still working on the 300B filament supply. I should be done shortly. Then I'll repost the schematics for the whole enchilada.

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We actually already settled on the PTFWF460 (460-0-460)@200mA transformer.

However, the link you posted is great for anyone ordering Hammond transformers in the US. Free shipping on orders over $80! I think that's every Hammond order I've ever made.

I've used AES (http://www.tubesandmore.com) for Hammond iron, they didn't have any 300 series over 800VCT. That link posted was the only site I found that carried the 300 series up to the1000VCT. It's easy to exceed that $80 with Hammond iron.

Can't wait to see the finished design. Sounds really great!

_________________The key to a successful build is to keep the smoke IN the circuit. -Les

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